It is known that by color-developing a silver halide color photographic material, coupler(s) in the photographic material react with an oxidized aromatic primary amino color developing agent to form indophenol, indoaniline, indamine, azomethine, phenoxazine, and similar dyes, whereby color images are formed.
In the system, a subtractive color process is usually used for color reproduction and silver halide emulsions each being selectively sensitive to blue, green or red and yellow, magenta, and cyan color image-forming agents (couplers) which are in complementary color relation with the above-described silver halide emulsions, respectively are used.
Various studies have hitherto been made to bring about improvement of the performance of couplers for forming imaging dyes. As disclosed in many patents and technical literatures, the performance of couplers which are used for the above-described color photographic materials produces large influences on the general photographic performance of the color photographic materials such as sensitivity, color reproducibility, sharpness, etc.; the image quality thereof such as graininess, etc.; and the image storability to heat, light, humidity, etc. For example, if the dye formed by the coupling reaction of a coupler and the oxidation product of a developing agent has a broad spectral absorption width or an unnecessary side absorption, color turbidity occurs to reduce the color reproducibility. Also, as the amount of a dye capable of being formed from a definite amount of coupler is larger, the amount of the coupler to be added to a silver halide emulsion layer can be reduced, which results in reducing the thickness of the emulsion layer, inhibiting the occurrence of color ooze by an optical cause, and improving the sharpness of color images formed. Furthermore, as the stability of the dye formed from a coupler to heat, light, humidity, or ingredients contained in a color photographic material is higher, the storability of the imaging dye is higher. Moreover, as the rate of the coupling reaction of a coupler and the oxidation product of a developing agent is higher, the development proceeds more efficiently and the sensitivity of the light-sensitive material becomes higher.
Practical examples of couplers for improving the photographic performance, as described above are discussed in more detail below.
That is, pyrazolone type couplers capable of forming azomethine dyes having a reduced side absorportion at the short wave length are reported in Journal of Chemical Society, Perkin I, page 2047 (1977). Also, bis-type yellow couplers having a reduced molecular weight necessary for forming a definite density of a dye is disclosed in U.S. Pat. No. 4,248,961. Furthermore, couplers giving color images having improved fastness are proposed in U.S. Pat. Nos. 3,311,476 and 4,124,396. Still further, couplers showing a high coloring property useful for increasing the sensitivity are disclosed in U.S. Pat. Nos. 4,296,199 and 4,301,235.
As described above, the improvement of the performance of couplers has recently been proposed but it is not always satisfactory. On the contrary, it can be said to be difficult to remarkedly improve the performance of couplers owing to the restriction inevitably occurring so far as conventional couplers are used. The inevitable restriction of a conventional coupler is that the coupler itself has a function of "receiving an imaging information accumulated in silver halides through the oxidation product of a developing agent to become a chromophore component of imaging dye". Accordingly, the property of the imaging dye formed from a coupler is determined by the molecular structure of the coupler, whereby there are various discrepancies on the construction of coupler molecules.
For example, a coupler having a molecular structure showing a high coupling rate forms an imaging dye having an undesirable spectral absorption or forms an imaging dye having a large side absorption. Also, a coupler capable of forming a dye having high fastness shows a low coupling rate. Furthermore, a coupler forming a dye having a sharp spectral absorption width and less side absorption has a problem as to the stability thereof. In other words, the guide lines for selecting a coupler having an efficient coloring function are not always consistent with the guide lines for selecting a coupler having a function as a desired imaging dye.
Under these circumstances, dye-releasing timing-type couplers have been proposed in Japanese Patent Application (OPI) No. 145135/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") (corresponding to British Patent Application No. 2,010,818A), wherein the hue of an anion structure of a hydroxy-substituted aromatic azo dye formed by development process is utilized.
However, since the pH of a photographic film after being processed by an ordinary development process is weakly acidic, the above-described azo dye released by the coupling reaction of the coupler and the oxidation product of an aromatic primary amine developing agent is partially achromatized by protonation, which results in the reduction of image density. Moreover, since a cation which pairs with the azo dye is a metal ion in the developer, the preservability of the images formed is insufficient. In the invention of the above-described patent application, for solving these problems, a specific process of using a stabilization bath of a high pH (about 11.0) containing cetyltrimethylammonium bromide as a final bath is required for keeping the hue of the azo dye formed. As a result, the above-described invention has problems for practical use.